Analytically accurate, clinically inaccurate: A diagnostic case of Pseudohyperparathyroidism

Laboratory results are central to the diagnosis and management of endocrine disorders, but preanalytical factors may lead to misleading findings and inappropriate clinical interpretation. We report a 33-year-old kidney transplant recipient with a history of secondary hyperparathyroidism treated by parathyroidectomy several years earlier, who presented with an unexpectedly elevated parathyroid hormone (PTH) concentration exceeding the analytical measurement range (AMR) despite normal serum calcium and phosphorus. Given the marked biochemical discordance, a structured investigation was undertaken to exclude assay-related causes. Serial dilution studies demonstrated linear, proportional recovery consistent with negative controls. Polyethylene glycol (PEG) precipitation and heterophile antibody blocking studies failed to reduce PTH into the AMR, making significant immunoassay interference unlikely. With analytical causes excluded, investigation focused on preanalytical variables and ultimately demonstrated a collection-site-dependent elevation in PTH rather than true systemic hypersecretion, confirmed on repeat sampling from an alternative site. This finding was particularly relevant in the context of prior parathyroid autotransplantation and limited vascular access. This case highlights that analytical validity does not always equate to clinical validity and underscores the importance of specimen collection site in the interpretation of PTH results. Close collaboration between clinicians and the clinical laboratory is essential to recognize preanalytical sources of discordant results and avoid misinterpretation.